1. Field of the Invention
The present invention relates to an electric power steering device, assisting a steering force applied by a driver of an automobile and so on by a motor.
2. Discussion of Background
Various methods of protecting an electric power steering device assisting a steering force, applied by a driver, by a motor are conventionally devised. FIGS. 16 and 17 illustrate a conventional electric power steering device disclosed in Japanese Utility Model No. 2586020.
FIG. 16 is a control block chart illustrating the conventional electric power steering device.
In FIG. 16, numerical reference 1 designates a torque sensor detecting a steering force, applied by a driver; numerical reference 2 designates a speed sensor detecting a speed of a vehicle; numerical reference 3 designates a microprocessor; numerical reference 4 designates a motor driving circuit; numerical reference 5 designates a motor, driven by the motor driving circuit 4 to generate a steering assisting force; and numerical reference 6 designates a motor current detecting means detecting a current flowing through the motor 5. Numerical reference 31 designates a steering force assisting current setting means determining the motor current in order to reduce the steering force by the driver; numerical reference 32 designates an inertia compensating current setting means determining the motor current in order to reduce an influence of a moment of inertia of the motor; and numerical reference 33 designates an upper limit motor current setting means determining an upper limit of the motor current in order to protect the motor driving circuit 4 from overheat and to maintain the motor current, wherein numerical references 31 through 33 are realized by a software in the microprocessor 3.
FIG. 17 illustrates the upper limit motor current of the conventional electric power steering device.
Next, an operation of the conventional electric power steering device will be described.
When the driver steers a steering wheel, a steering force is detected by a torque sensor 1, and a signal is inputted in the microprocessor 3. The microprocessor 3 sets the steering force assisting current in the steering force assisting current setting means 31 to obtain an appropriate steering force based on a vehicle speed detected by the vehicle speed sensor 2 and the steering force. Further, the inertia compensating current is set by the inertia compensating current setting means 32 in order to reduce an influence of the moment of inertia of the motor and to improve a steering feeling. The steering force assisting current is limited to be the upper limit value or less, wherein the upper limit value is determined in accordance with a characteristic illustrated in FIG. 17 in response to an integrated value of the motor current, detected by the motor current detecting means 6, squared. Thus limited steering force assisting current and the inertia compensating current are added and fed for a feedback control so that the added value and the detected value of the motor current by the motor current detecting means 6 match. The motor 5 is driven by the motor driving circuit 4.
In the conventional electric power steering device, a squared value of the current has a close relationship with a calorific value and is appropriate for an index of overheat protection. The upper limit of the motor current is determined in response to the integrated value of the motor current squared in the motor current upper limit value setting means 33. However, a loss in heat generating portions of the motor and the controller is analoguous to a power function of the current, and an exponent of the power function is between the first power and the second power. Accordingly, especially in a large current range, when the overheat protection is conducted using the index of the current squared, there is a problem that an overheat is excessively protected. As a result, in case of parking a vehicle in a garage located in a narrow parking area by stationarily steering the steering wheel, there are problems that the steering assisting force becomes small, and the steering force by the driver is increased.
Another conventional device, which determines an upper limit of a motor current in response to an integrated value of the motor current to the first power, is also known. In this case, as disclosed in Japanese Utility Model No. 2586020, the upper limit value is not rational, and it is necessary to design the motor driving circuit 4 with a margin.
Hereinbelow, another conventional device will be described with reference to the figures.
FIG. 18 illustrates an equivalent circuit of a generally used d.c. motor.
In FIG. 18, numerical reference 7 designates a resistance of an armateur; numerical reference 8 designates an inductance of the armateur; and numerical reference 9 designates a resistance of a brush.
FIG. 19 illustrates a voltage drop in the brush of the d.c. motor illustrated in FIG. 18.
In FIG. 18, provided that the motor current is represented by Im, and the voltage drop in the brush is represented by Vbr, a copper loss Pm of the motor is expressed by the following equation.
Pm=Ra*Im2+Vbr*Im,xe2x80x83xe2x80x83(Equation 1)
where
Pm denotes the copper loss of the motor (W);
Ra denotes the resistance of the armateur (xcexa9);
Im denotes the current of the armateur (A); and
Vbr denotes the voltage drop in the brush (V).
As illustrated in FIG. 19, the voltage drop Vbr in the brush increases as the current Im of the armateur increases. When the current Im of the armateur becomes a predetermined value Im1 or more, the voltage drop is saturated at a predetermined value Vbr1. In a large current range that current of armateur Im greater than  predetermined value Im1, where a heat from the motor causes problems, the voltage drop Vbr in the brush becomes constant irrespective of the current Im of the armateur.
From FIG. 19 and Equation 1, it is possible to regard the copper loss Pm of the motor a sum of a term in proportion to the current Im squared of the amateur and a term in proportion to the current Im of the armateur to the first power. Therefore, the copper loss Pm of the motor is a power function of the current Im of the armateur as follows.
Pm≈C1*Imn1,xe2x80x83xe2x80x83(Equation 2)
Where
1xe2x89xa6n1xe2x89xa62, and
C1 denotes an arbitrary constant.
Thus the copper loss Pm of the motor is analoguous to Equation 2.
FIG. 20 illustrates the motor driving circuit of a conventional electric power steering controller.
In FIG. 20, numerical reference 4 designates a motor driving circuit composed of MOSFET Q1 through Q4; numerical reference 5 designates a motor; and numerical reference 10 designates a battery.
FIG. 21 is a graph illustrating a waveform of a motor current of the motor driving circuit illustrated in FIG. 20, wherein MOSFET Q1 and Q4 are driven for PWM, and MOSFET Q2 and Q3 are turned off.
FIG. 22 illustrates a voltage drop of a parasitic diode MOSFET of the motor driving circuit in the conventional electric power steering device.
Next, an operation of the motor driving circuit illustrated in FIG. 20 will be described. In a duration that MOSFET Q1 and Q4 are turned on, the motor current flows through a passage I1. In a duration that MOSFET Q1 and Q4 are turned off, parasitic diodes of MOSFET Q2 and Q3 are turned on, whereby the motor current flows through a passage I2. Provided that losses of MOSFET Q1 through Q4 respectively are P1 through P4, and a switching loss is ignored, a loss Pd of the motor driving circuit 4 is expressed by following equations.
Pd=P1+P2+P3+P4xe2x80x83xe2x80x83(Equation 3)
P1=P4=xcex1*Ron*Im2xe2x80x83xe2x80x83(Equation 4)
xe2x80x83P2=P3=(1xe2x88x92xcex1)VF*Imxe2x80x83xe2x80x83(Equation 5)
where
Pd denotes a loss (W) without the switching loss of the motor driving circuit;
P1 denotes the loss (W) without the switching loss of MOSFET Q1;
P2 denotes the loss (W) without the switching loss of MOSFET Q2;
P3 denotes the loss (W) without the switching loss of MOSFET Q3;
P4 denotes the loss (W) without the switching loss of MOSFET Q4;
xcex1 denotes a flow rate of a current through MOSFET Q1 and Q4;
Ron denotes a resistance (xcexa9) at time of turning on MOSFET;
Im denotes a motor current (A), equals to I1 and I2; and
VF denotes a voltage (V) of a parasitic diode of MOSFET in a direction of easy flow.
As illustrated in FIG. 22, the voltage VF of the parasitic diode of MOSFET in the direction of easy flow increases as the motor current Im increases. When the motor current In becomes a predetermined value Im2 or more, the voltage VF saturates at a predetermined value VF1. In other words, in a large current range of Im greater than Im2, where a heat of the motor driving circuit 4 becomes a problem, the voltage VF of the parasitic diode of MOSFET in the directon of easy flow is constant regardless of the motor current Im.
In reference of FIG. 22 and Equations 3 through 5, the loss Pd without the switching loss of the motor driving circuit can be regarded as a sum of a term in proportion to the motor current Im squared and a term in proportion to the motor current Im to the first power. Therefore, the loss Pd without the switching loss of the motor driving circuit is approximately expressed as a power function of the motor current Im as follows.
Pd≈C2*Imn2,xe2x80x83xe2x80x83(Equation 6)
Where
1xe2x89xa6n2xe2x89xa62; and
C2 denotes an arbitrary constant.
As described, the losses of the motor and the controller approximate to power functions of the current, and indices of exponential function are between the first power and the second power. Therefore, overheat protection is excessive when the current to the second power is used as an indicator of the heat.
Further, when the upper limit of the motor current is determined in response to the integrated value of the motor current to the first power, setting of the upper limit value is not rational, and it is necessary to give a margin in designing the motor driving circuit 4.
It is an object of the present invention to solve the above-mentioned problems inherent in the conventional technique and to provide an electric power steering device, which is provided with appropriate overheat protection to maintain a sufficient steering assisting force even when stationary steering is repeated.
According to a first aspect of the present invention, there is provided an electric power steering device assisting a steering force by a motor, the electric power steering device comprising a motor current upper limit value setting means setting an upper limit value of a motor current based on a power function of the motor current, and indices of an exponential function of the power function are in a range of between 1 and 2.
According to a second aspect of the present invention, there is provided an electric power steering device assisting a steering force by a motor, the electric power steering device comprising a motor current upper limit value setting means for setting an upper limit value of a motor current based on a power function of a deviation between the motor current and a motor current reference value.
According to a third aspect of the present invention, there is provided an electric power steering device assisting a steering force by a motor, the electric power steering device comprising a motor current upper limit value setting means for setting an upper limit value of a motor current based on a deviation between a power function of the motor current and a reference value of the power function of the motor current.
According to a fourth aspect of the present invention, there is provided the electric power steering device, wherein indices of an exponent of the power function are in a range between 1 and 2.
According to a fifth aspect of the present invention, there is provided the electric power steering device further comprising a motor current detecting means detecting the motor current, wherein the motor current is a current detected by the motor current detecting means.
According to a sixth aspect of the present invention, there is provided the electric power steering device, wherein the power functions are approximate to a polynomial.
According to a seventh aspect of the present invention, there is provided the electric power steering device, wherein the power functions are approximate to a polygonal line graph.
According to an eighth aspect of the present invention, there is provided the electric power steering device further comprising a motor driving circuit, which drives the motor in a plurality of moods, wherein constants of the power functions are switched in response to driving modes of the motor driving circuit.
According to a ninth aspect of the present invention, there is provided the electric power steering device, wherein the power functions are delayed in time by a predetermined function to set the upper limit value of the motor current.
According to a tenth aspect of the present invention, there is provided the electric power steering device further comprising a temperature detecting means detecting temperatures of portions related to a temperature increment, wherein the motor current upper limit value setting means adjusts the upper limit value of the motor current in response to the temperatures detected by the temperature detecting means.
According to an eleventh aspect of the present invention, there is provided the electric power steering device, wherein the motor current upper limit value setting means operates a plurality of upper limit values of the motor current using a plurality of power functions, and selects one of the plurality of upper limit values of the motor current.